And that's because Captain America's adventures are probably consistently the most politically complex of any Marvel superhero.
I think Marvel has always known that Cap as a character is loaded with political significance, both due to his wartime popularity and the way he stands for America. Unlike a lot of other "patriotic" heroes who had cute names or historical inspirations, his name proclaims that he represents America. Which immediately
And writers for Marvel certainly figured this out relatively early. While Green Lantern was awkwardly trying to prove that he was concerned about the inner-city when not fighting Sinestro (in what reads now as a Guess Who's Coming to Dinner? argument for tolerance), writers of Cap grappled with more complex issues.
Steve Englehart's run in the early 70s (collected here and here), though sometimes clumsily and earnestly written, was an incredibly ambitious attempt to grapple with the problems of post-Vietnam/Watergate America. Cap acquired a black sidekick from the inner-city who disagreed with Cap's more conservative views. Cap encountered an insane anti-Communist imitator who embodied the worst of 1950s McCarthyism. And he faced a conspiracy led by the Committee to Re-establish America's Principles (just read the bolded words), a clear attack on Nixon's Committee to Re-Elect the President, which attacked Cap as a left-wing radical for his moderate views. Englehart's first major storyline culminated in a showdown at the White House where a shadowy figure in the Oval Office committed suicide to avoid paying for his crimes against the American people.
And in the 80s, while DC's British Invasion pushed forward the subject matter and tone of comics, Mark Gruenwald's run on Captain America and Squadron Supreme tried to do the same for Marvel's titles. While Squadron Supreme certainly deserves further examination, Gruenwald's attempts to engage with Reagan's "Morning in America" in Cap are also quite canny.
Gruenwald's most important political storyline was one where a Senate Subcommittee decided that Captain America's political loyalties were way too nuanced and unreliable and that Steve Rogers (Cap's alter-ego) had to be replaced by someone else. The irony that a man who fought Nazis in World War II was insufficiently patriotic for Reagan's America was subtly underplayed.
In the ensuing storyline, the Senate selects a character named the Superpatriot to replace Steve Rogers. He gets the job after all sorts of staged and falsely spun PR antics that show him as more "law and order" than Cap. As the storyline unfolded, John Walker (the new Cap) proved himself to be more brutal and less competent than his predecessor and easily manipulated by his enemies. In the end, it falls to Steve Rogers and his more compassionate/moderate left style to save the world where the jingoistic Reaganite asshole failed.
This barely scratches the surface of the ways Cap's been used to comment on America's view of itself and how we define the ideal American. Some of these stories are clunky, especially compared to the more stripped-down "realist" style most superhero comics prefer these days, but they do make ambitious attempts to grapple with America the ideal versus America the reality.
Englehart uses Cap to grapple with Watergate.
(Thanks, Paul Constant for the image.)